1. Field of the Invention
The present invention generally relates to the field of inspection and analysis of specimens and, more particularly, to defect inspection and analysis of semiconductor integrated circuits.
2. Description of the Prior Art
In the semiconductor integrated circuit (IC) industry, there is a continuing demand for higher circuit packing densities. This demand of increased packing densities has led the semiconductor industry to develop new materials and processes to achieve sub-micron device dimensions. Manufacturing IC""s at such minute dimensions adds more complexity to circuits and the demand for improved methods to inspect integrated circuits in various stages of their manufacture is ever present.
Although inspection of such products at various stages of manufacture is very important and can significantly improve production yield and product reliability, the increased complexity of IC""s increases the cost of such inspections, both in terms of expense and time. However, if a defect can be detected early in production, the cause of the defect can be determined and corrected before a significant number of defective IC""s are manufactured.
In order to overcome the problems posed by defective IC""s, IC manufacturers sometimes fabricate semiconductor defect test structures. Such defect test structures are dedicated to defect analysis. The defect test structures are fabricated such that they are sensitive to defects that occur in IC product, but are designed so that the presence of defects is more readily ascertained. Such defect test structures are often constructed on the same semiconductor substrate as the IC products.
One example of a defect test structure is found in the Copper CMP Test Mask Set designed at MIT. This test mask set is designed to quantify the dependence of the resulting copper line profile on parameters such as line pitch, line width and line aspect ratio. However, the MIT mask set is designed to be probed using conventional electrical testing in which current is passed through the device by contacting predefined pad of large area (approximately 100xc3x97100 xcexcm2) with electrical probes, not by electron beam. As is well known in the art, defect detecting systems frequently utilize charged particle beams. In such systems, a charged particle beam, such as an electron beam, is irradiated on defect test structures. The interaction of the electron beam with features in the circuitry generates a number of signals in varying intensities, such as secondary electrons, back-scattered electrons, x-rays, etc. Typically, electron beam methods employ secondary electron signals for the well known xe2x80x9cvoltage contrastxe2x80x9d technique for circuit defect detection.
The voltage contrast technique operates on the basis that potential differences in the various locations of a test structure under examination cause differences in secondary electron emission intensities. Thus, the potential state of the scanned area is acquired as a voltage contrast image such that a low potential portion of, for example, a wiring pattern might be displayed as bright (intensity of the secondary electron emission is high) and a high potential portion might be displayed as dark (lower intensity secondary electron emission). Alternatively, the system may be configured such that a low potential portion might be displayed as dark and a high potential portion might be displayed as bright.
A secondary electron detector is used to measure the intensity of the secondary electron emission that originates only at the path swept by the scanning electron beam. A defective portion can be identified from the potential state of the portion under inspection. In one form of inspection, the mismatched portion between the defective voltage contrast image and the defect free one reveals the defect location.
Thus, in such systems, the voltage contrast is simultaneously monitored for both defective and defect free circuits for each circuit manufactured. However, considering the density of IC""s currently produced, the time necessary to scan voltage contrast data to perform comparisons is significant. The inspection and analysis of such circuits may take several days. Accordingly, more efficient voltage contrast inspection systems are desirable.
The present invention includes a system for detecting defects in test structures. The system operates so as to provide efficient and effective testing of defects. It also includes novel test structures that provide for improved defect testing, as are described more fully below.
In one embodiment, the invention pertains to a method of inspecting a sample. The method includes moving to a first field associated with a first group of test structures. The first group of test structures are partially within the first field. The method further includes scanning the first field to determine whether there are any defects present within the first group of test structures. When it is determined that there are defects within the first group of test structures, the method further includes repeatedly stepping to areas and scanning such areas so as to determine a specific defect location within the first group of test structures. In one implementation, the defect is located outside the inspected first field.
In another embodiment, a test structure is disclosed. The test structure has a plurality of conductive lines each having a first end and a second end. The first ends of the conductive lines are within a scannable area, and the second ends of the conductive lines extend away from the scannable area. At least one of the conductive lines is either floating or coupled to a predetermined voltage potential. Preferably, the predetermined voltage potential is at a nominal ground potential. The conductive lines alternate between floating and being coupled to the predetermined voltage potential. In another embodiment, the second ends of conductive lines are coupled to the predetermined voltage potential. In another aspect, the conductive lines cross from outside a perimeter of the scannable area to inside the perimeter. In yet another embodiment, the test structures are primarily located outside of the scannable area.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.